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Allow non-constant calls to constexpr functions. (#7067)

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These turn up frequently in real-world code, for example when converting
a mutable global `Cpp.std.string_view` to a `Cpp.std.string`. Only
reject a non-constant call if the callee is `consteval`, not if it's
`constexpr`.
This commit is contained in:
Richard Smith
2026-04-16 17:10:48 +00:00
committed by GitHub
parent cdfa57f230
commit 1d5113649b
2 changed files with 58 additions and 10 deletions
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toolchain/check/testdata/interop/cpp/constexpr.carbon
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+54 -9
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@@ -3,6 +3,7 @@
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/full.carbon
// EXTRA-ARGS: --clang-arg=--std=c++20
//
// AUTOUPDATE
// TIP: To test this file alone, run:
@@ -120,14 +121,58 @@ constexpr int f(int a) {
}
''';
// CHECK:STDERR: fail_invalid_constant_eval.carbon:[[@LINE+10]]:26: error: call to immediate function 'f' is not a constant expression [CppInteropParseError]
// CHECK:STDERR: 21 | let a: array(i32, Cpp.f(5)) = (1, 2, 3);
// CHECK:STDERR: | ^
// CHECK:STDERR: fail_invalid_constant_eval.carbon:[[@LINE-7]]:10: note: cannot refer to element 5 of array of 1 element in a constant expression [CppInteropParseNote]
// CHECK:STDERR: 7 | return arr[a];
// CHECK:STDERR: | ^
// CHECK:STDERR: fail_invalid_constant_eval.carbon:[[@LINE+4]]:26: note: in call to 'f(5)' [CppInteropParseNote]
// CHECK:STDERR: 21 | let a: array(i32, Cpp.f(5)) = (1, 2, 3);
// CHECK:STDERR: | ^
// CHECK:STDERR: fail_invalid_constant_eval.carbon:[[@LINE+4]]:19: error: array bound is not a constant [InvalidArrayExpr]
// CHECK:STDERR: let a: array(i32, Cpp.f(5)) = (1, 2, 3);
// CHECK:STDERR: ^~~~~~~~
// CHECK:STDERR:
let a: array(i32, Cpp.f(5)) = (1, 2, 3);
// --- nonconstant_constexpr_call.carbon
library "[[@TEST_NAME]]";
import Cpp;
var a: i32 = 0;
inline Cpp '''
constexpr int f(const int &r) { return r; }
''';
// OK, runtime call.
var b: i32 = Cpp.f(a);
// --- fail_nonconstant_consteval_call.carbon
library "[[@TEST_NAME]]";
import Cpp;
var a: i32 = 0;
// TODO: This is failing when building the thunk, not when calling it.
inline Cpp '''
// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+9]]:15: error: call to consteval function 'f' is not a constant expression [CppInteropParseError]
// CHECK:STDERR: 19 | consteval int f(const int &r) { return r; }
// CHECK:STDERR: | ^
// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+6]]:15: note: function parameter 'r' with unknown value cannot be used in a constant expression [CppInteropParseNote]
// CHECK:STDERR: 19 | consteval int f(const int &r) { return r; }
// CHECK:STDERR: | ^
// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+3]]:15: note: declared here [CppInteropParseNote]
// CHECK:STDERR: 19 | consteval int f(const int &r) { return r; }
// CHECK:STDERR: | ^
consteval int f(const int &r) { return r; }
''';
// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+11]]:14: note: in thunk for C++ function used here [InCppThunk]
// CHECK:STDERR: var b: i32 = Cpp.f(a);
// CHECK:STDERR: ^~~~~~~~
// CHECK:STDERR:
// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+7]]:14: error: non-constant call to compile-time-only function [NonConstantCallToCompTimeOnlyFunction]
// CHECK:STDERR: var b: i32 = Cpp.f(a);
// CHECK:STDERR: ^~~~~~~~
// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE-10]]:15: note: compile-time-only function declared here [CompTimeOnlyFunctionHere]
// CHECK:STDERR: consteval int f(const int &r) { return r; }
// CHECK:STDERR: ^
// CHECK:STDERR:
var b: i32 = Cpp.f(a);